Method for making lithium-ion battery anodes

ABSTRACT

The present disclosure relates to a method for making a lithium-ion battery anode. The method comprises the steps of scratching off a carbon nanotube array to obtain a plurality of carbon nanotubes, adding the plurality of carbon nanotubes into a solvent, and ultrasonically dispersing the solvent to make the plurality of carbon nanotubes form a three-dimensional network-like structure; adding a titanium salt into the solvent, wherein the titanium salt hydrolyzes to form a plurality of titanium dioxide particles, and the plurality of titanium dioxide particles are adsorbed on surfaces of the plurality of carbon nanotubes; and separating the nanotube three-dimensional network structure from the solvent to form a precursor, and drying the precursor to form a titanium dioxide-carbon nanotube composite film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. § 119 fromChina Patent Application No. 201710272313.6, filed on Apr. 24, 2017, inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference. The application is also related to copendingapplications 15/891,442 entitled, “LITHIUM-ION BATTERY ANODES ANDFLEXIBLE LITHIUM-ION BATTERIES USING THE SAME”, filed Feb. 8, 2018.

FIELD

The present disclosure relates to a method for making lithium-ionbattery anodes.

BACKGROUND

Carbon nanotubes (CNTs)-titanium dioxide (TiO₂) composites using as alithium battery anode could avoid a collapse of electrodes and suppressa formation of solid electrolyte interface (SEI) layers and lithiumdendrites. The lithium battery anode comprises the carbon nanotubes(CNTs)-Titanium dioxide (TiO₂) composites has excellent electricalconductivity and high-rate current characteristics.

However, conventional methods for making lithium battery anodescomprising the carbon nanotubes (CNTs)-titanium dioxide (TiO₂)composites need high temperature heating, and require strong acids orsurfactants to modify a surface of carbon nanotube. Therefore,conventional methods are easy to introduce impurities and damage carbonnanotubes.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present technology will now be described, by wayof example only, with reference to the attached figures, wherein:

FIG. 1 is a scanning electron microscope image of one embodiment of alithium-ion battery anode.

FIG. 2 is a structure schematic view of one embodiment of a lithium-ionbattery anode.

FIG. 3 is a scanning electron microscope image of one embodiment of alithium-ion battery anode in which a plurality of titanium dioxidenanoparticles coat on a surface of a plurality of carbon nanotubes toform a continuous titanium dioxide layer.

FIG. 4 is a scanning electron microscope image of one embodiment of alithium-ion battery anode in which a plurality of titanium dioxidenanoparticles is uniformly dispersed on a surface of a plurality ofcarbon nanotubes.

FIG. 5 is charge and discharge cycles curves at different rates of alithium-ion battery anode of Example 1 and a lithium-ion battery anodeof Comparative Example 1.

FIG. 6 is a constant current charge-discharge curve of one embodiment ofa lithium-ion battery anode.

FIG. 7 is a scanning electron microscope image of one embodiment of alithium-ion battery anode after charged and discharged 1000 times at aconstant rate of 60 C.

FIG. 8 is a flow chart of one embodiment of making a lithium-ion batteryanode.

FIG. 9 is a flow chart of one embodiment of making a lithium-ion batteryanode.

FIG. 10 is a flow chart of one embodiment of making a lithium-ionbattery anode.

FIG. 11 is a structure schematic view of one embodiment of a flexiblelithium-ion battery.

FIG. 12 is charge and discharge cycles curves at different rates of aflexible lithium-ion battery of Example 3.

FIG. 13 is a fast charging curve of the flexible lithium-ion battery ofExample 3.

FIG. 14 is a comparison of capacity retention before and after bendingof the flexible Li-ion battery of Example 3.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean “at leastone.”

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures, and components havenot been described in detail so as not to obscure the related relevantfeature being described. Also, the description is not to be consideredas limiting the scope of the embodiments described herein. The drawingsare not necessarily to scale, and the proportions of certain parts havebeen exaggerated to illustrate details and features of the presentdisclosure better.

Several definitions that apply throughout this disclosure will now bepresented.

The term “substantially” is defined to be essentially conforming to theparticular dimension, shape, or other feature which is described, suchthat the component need not be exactly or strictly conforming to such afeature. The term “comprise,” when utilized, means “include, but notnecessarily limited to”; it specifically indicates open-ended inclusionor membership in the so-described combination, group, series, and thelike.

All words of approximation as used in the present disclosure and claimsshould be construed to mean “approximate,” rather than “perfect,” andmay accordingly be employed as a meaningful modifier to any other word,specified parameter, quantity, quality, or concept. Words ofapproximation, include, yet are not limited to terms such as“substantial”, “nearly”, “almost”, “about”, “generally”, “largely”,“essentially”, “closely approximate”, etc.

Referring to FIG. 1 and FIG. 2, one embodiment is described in relationto a lithium-ion battery anode 100. The lithium-ion battery anode 100comprises a carbon nanotube film 10 and a plurality of titanium dioxide(TiO₂) nanoparticles 20. In one embodiment, the lithium-ion batteryanode 100 consists of the carbon nanotube film 10 and the plurality oftitanium dioxide nanoparticles 20. The carbon nanotube film 10 is aflexible and free-standing carbon nanotube film. The carbon nanotubefilm 10 comprises a plurality of carbon nanotubes 12 uniformlydistributed in the carbon nanotube film 10. The plurality of titaniumdioxide nanoparticles 20 are uniformly adsorbed on the surfaces of theplurality of carbon nanotubes 12. A particle size of each of theplurality of titanium dioxide nanoparticles 20 is less than or equal tothirty nanometers. The lithium-ion battery anode 100 has flexibility andfree-standing property, thus, the lithium-ion battery anode 100 can bebent freely.

In one embodiment, the particle size of each of the plurality oftitanium dioxide nanoparticles 20 is in a range from about 3 nanometersto about 20 nanometers. In another embodiment, the particle size of eachof the plurality of titanium dioxide nanoparticles 20 is in a range fromabout 3 nanometers to about 10 nanometers. When the particle size ofeach of the plurality of titanium dioxide nanoparticles 20 is in suchranges, the plurality of titanium dioxide nanoparticles 20 has largecontact areas with electrolyte and Li⁺ ions, and offers more lithiumstorage sites; thus, when the particle size of each of the plurality oftitanium dioxide nanoparticles 20 is in such ranges, the lithium-ionbattery anode 100 has high capacities even at very high charge/dischargerates.

The carbon nanotube film 10 is a flexible and free-standing carbonnanotube film. “Free-standing” means that the carbon nanotube film 10can maintain a film structure, and not change even if part of the carbonnanotube film 10 is located on a support. For example, if the carbonnanotube film 10 is placed on a frame or two separate supports, part ofthe carbon nanotube film 10 not in contact with the frame or twoseparate supporting elements would be self-suspended between parts ofthe frame or between two supporters and maintain their film structureintegrity. The plurality of carbon nanotubes 12 are joined end-to-endsubstantially along the same direction and connected to each other byvan der Waals attractive force therebetween, thereby forming theflexible and free-standing carbon nanotube film. A thickness of thecarbon nanotube film 10 can be selected according to actual needs. Inone embodiment, the thickness of the carbon nanotube film 10 ranges fromabout 100 nanometers to about 100 micrometers.

The carbon nanotube film 10 comprises a plurality of micro pores. Theplurality of micro pores improves a penetration of the electrolyte, andan ability of the lithium-ion battery anode 100 to adsorb and removelithium-ions; and the plurality of micro pores can be used to holdlithium-ions. Therefore, a volume of the lithium-ion battery anode 100will not be significantly increased during use.

The plurality of carbon nanotubes 12 can be single-walled carbonnanotubes, double-walled carbon nanotubes, or multi-walled carbonnanotubes. A particle size of each of the plurality of carbon nanotube12 can be in a range from about 1 nanometer to about 200 nanometers. Inone embodiment, the particle size of each of the plurality of carbonnanotube is in a range from about 20 nanometers to about 30 nanometers.The plurality of carbon nanotubes 12 can be pure, meaning there are fewor no impurities adhered on surface of the plurality of carbon nanotubes12. In one embodiment, a length of each of the plurality of carbonnanotube 12 is longer than 300 micrometers.

The plurality of titanium dioxide nanoparticles 20 can bediscontinuously dispersed on the surface of each of the plurality ofcarbon nanotubes 12. The plurality of titanium dioxide nanoparticles 20can also continuously coat on the surface of each of the plurality ofcarbon nanotubes 12, to form a continuous titanium dioxide layer on thesurface of each of the plurality of carbon nanotubes 12. Referring toFIG. 3, in one embodiment, the titanium dioxide nanoparticles 20 coat onthe surface of each of the plurality of carbon nanotubes 12 to form acontinuous titanium dioxide layer, and a core-shell structure is formedby the continuous titanium dioxide layer and the plurality of carbonnanotubes 12. If a thickness of the continuous titanium dioxide layer istoo large, such as larger than 20 nanometers, the titanium dioxidenanoparticles 20 and the electrolyte can not be fully contacted, thus,an electrochemical performance of the lithium-ion battery anode 100 willbe reduced; on the contrary, if the thickness of the continuous titaniumdioxide layer is too small, such as smaller than 3 nanometers, an activematerial of the lithium-ion batteries is too less to result in poorlithium storage performance, a capacity of the lithium-ion battery willdrop. In one embodiment, the thickness of the continuous titaniumdioxide layer ranges from about 3 nanometers to about 20 nanometers. Inanother embodiment, the thickness of the continuous titanium dioxidelayer is about 10 nanometers.

Referring to FIG. 4, in one embodiment, the titanium dioxidenanoparticles 20 are uniformly dispersed on the surface of the pluralityof carbon nanotubes 12, and the plurality of titanium dioxidenanoparticles 20 do not agglomerate on the surface of the plurality ofcarbon nanotube 12. Therefore, an electron shuttle distance is short, anion transport speed and electron transport rate are high, the batterymagnification is small, and the battery attenuation is slow. In oneembodiment, a spacing between adjacent titanium dioxide nanoparticles ofthe plurality of titanium dioxide nanoparticles 20 is ranged from about3 nanometers to about 20 nanometers. If the spacing between adjacenttitanium dioxide nanoparticles 20 is too large, such as larger than 20nanometers, the active material of the lithium-ion batteries is too lessto result in poor lithium storage performance, and the capacity of thelithium-ion battery will drop; on the contrary, if the spacing betweenadjacent titanium dioxide nanoparticles 20 is too small, such as lessthan 3 nanometers, the titanium dioxide particles 20 will join togetherto form larger particles, the titanium dioxide nanoparticles 20 and theelectrolyte can not be fully contacted, thus, the active sites will bereduced.

A mass percentage of the titanium dioxide nanoparticles 20 in thelithium-ion battery anode 100 can range from about 20% to about 60%. Insuch range, the lithium-ion battery using the lithium-ion battery anode100 has small attenuation ratio and large capacity, and can achieve highrate ultrafast charge and discharge. If the mass percentage is toosmall, such as less than 20%, the active material of the lithium-ionbatteries is too less to result in poor lithium storage performance, andthe capacity of the lithium-ion battery will drop; on the contrary, ifthe mass percentage is too large, such as larger than 60%, the electronshuttle distance will be long, the battery magnification will be small,and the battery attenuation is fast. In one embodiment, the masspercentage of the titanium dioxide nanoparticles 20 in the lithium-ionbattery anode 100 is about 28.7%.

A mass density of the titanium dioxide nanoparticles 20 can be rangedfrom about 1 mg/cm² to about 3 mg/cm². The mass density of the titaniumdioxide nanoparticles 20 means a mass of the titanium dioxidenanoparticles 20 on one square centimeter of carbon nanotube surface. Ifthe mass density is too small, such as less than 1 mg/cm², the activematerial of the lithium-ion batteries is too less to result in poorlithium storage performance, and the capacity of the lithium-ion batterywill drop; on the contrary, if the mass density is too large, such aslarger than 3 mg/cm², the electron shuttle distance will be long, thebattery magnification will be small, and the battery attenuation will befast. In one embodiment, the mass density of the titanium dioxidenanoparticles 20 is about 1.3 mg/cm².

The plurality of carbon nanotubes 12 of the carbon nanotube film 10 arepure, and the carbon nanotube film 10 has a large viscosity, thus, thetitanium dioxide nanoparticles 20 can be absorbed on the carbon nanotubefilm 10 through physical force, such as a Ti—C noncovalent bondedinteraction.

Example 1

In this example, the lithium-ion battery anode 100 is assembled in afirst half cell, with pure lithium foils as a counter and referenceelectrode.

Comparative Example 1

In this comparative example, conventional lithium-ion battery anode isassembled in a second half cell, with pure lithium foils as a counterand reference electrode. The second half cell is the same as the firsthalf cell in Example 1, except that the lithium-ion battery anode isdifferent from the lithium-ion battery anode 100.

Referring to FIG. 5, when a cut-off voltage is in a range from about 1.0V to about 3.0V, the first half cell of Example 1 is discharged atcurrent densities of 1 C, 5 C, 10 C, 20 C, 30 C and 60 C, respectively.It can be seen that the first half cell demonstrate discharge capacitiesof 190 mA h g⁻¹, 145 mA h g⁻¹, 135 mA h g⁻¹, 125 mA h g⁻¹, 120 mA h g⁻¹,100 mA h g⁻¹ at 1 C, 5 C, 10 C, 20 C, 30 C and 60 C, respectively; andthe discharge capacity is still as high as 175 mA h g⁻¹ when the currentdensity returns to 1 C. However, the discharge capacities of the secondhalf cell at current densities of 1 C, 5 C, 10 C, 20 C, 30 C and 60 Care less than 25 mA h g⁻¹. Therefore, compared with conventionallithium-ion battery anode, the lithium-ion battery anode 100 has largerdischarge capacity and smaller battery capacity decay ratio.

Referring to FIG. 6, the first half cell of Example 1 is charged anddischarged at a constant rate of 60 C, after a charge/discharge cycle isperformed 1000 times, a capacity of the lithium-sulfur battery ofExample 1 is about 100 mA h g⁻¹, a capacity retention ratio of the firsthalf cell in Example 1 is larger than 95%, and a coulomb efficiency ofeach charge and discharge cycle is close to 100%. It can be seen thatthe capacity retention ratio and the capacity of the lithium-ion batteryanode 100 are high. Referring to FIG. 7, in one embodiment, after thelithium-ion battery anode 100 is charged/discharged 1000 times at aconstant rate of 60 C, a structure of the lithium-ion battery anode 100is substantially unchanged.

The lithium-ion battery anode 100 can have many advantages. First, thelithium-ion battery anode 100 has flexibility and free-standingproperty, and can be bent freely; and the particle size of each of theplurality of titanium dioxide nanoparticles 20 is small, which let thelithium-ion battery anode 100 has high capacities even at very highcharge/discharge rates. Second, the lithium-ion battery anode 100without any binders, conductive additives and current collectors, thus,under the same specific capacity and total capacity, the lithium-ionbattery using the lithium-ion battery anode 100 has a smaller mass;there are not insulation materials between the titanium dioxidenanoparticles 20, thus, the conductivity of the lithium-ion batteryanode 100 is increased; and the conductivity of the lithium-ion batteryanode 100 is more environmentally friendly. Third, the carbon nanotubefilm 10 comprises a plurality of micro pores, the plurality of micropores improves a penetration of the electrolyte and an ability of thelithium-ion battery anode 100 to adsorb and remove lithium-ions, and theplurality of micro pores can be used to hold lithium-ions, therefore, avolume of the lithium-ion battery anode 100 will not be significantlyincreased during use.

Referring to FIG. 8 and FIG. 9, an embodiment is described in relationto a first method for making the lithium-ion battery anode 100. Thefirst method embodiment comprises at least the following, general steps:

step (S1), providing a carbon nanotube array on a substrate, a solventand a titanium salt;

step (S2), scratching off the carbon nanotube array from the substrateto obtain a plurality of carbon nanotubes, adding the plurality ofcarbon nanotubes into the solvent, and ultrasonically dispersing thesolvent having the plurality of carbon nanotubes to make the pluralityof carbon nanotubes form a three-dimensional network-like structure;

step (S3), adding the titanium salt into the solvent with thethree-dimensional network-like structure, wherein the titanium salthydrolyzes to form a plurality of titanium dioxide particles, and theplurality of titanium dioxide particles are adsorbed on surfaces of theplurality of carbon nanotubes in the three-dimensional network-likestructure; and

step (S4), separating the nanotube three-dimensional network structureadsorbed with the plurality of titanium dioxide nanoparticles from thesolvent to form a precursor, and drying the precursor to form a titaniumdioxide-carbon nanotube composite film, wherein the titaniumdioxide-carbon nanotube composite film is the lithium-ion battery anode.

In more detail, in step (S1), the carbon nanotubes in the carbonnanotube array can be orderly or disorderly arranged. The term ‘orderlyarranged’ refers to the carbon nanotubes are arranged in a consistentlysystematic manner, e.g., the carbon nanotubes are arranged approximatelyalong a same direction and/or have two or more sections within each ofwhich the carbon nanotubes are arranged approximately along a samedirection (different sections can have different directions). The term‘disorderly arranged’ refers to the carbon nanotubes are arranged alongdifferent directions, and the aligning directions of the carbonnanotubes are random. In one embodiment, the carbon nanotube array is asuper-aligned carbon nanotube array. A length of the carbon nanotubes inthe super-aligned carbon nanotube array is long, such as larger than 300micrometers. The carbon nanotubes in the super-aligned carbon nanotubearray are pure, meaning there are few or no impurities adhered onsurface of the carbon nanotubes. The carbon nanotubes in thesuper-aligned carbon nanotube array are arranged along the samedirection.

A method for making the super-aligned carbon nanotube array can bechemical vapor deposition, arc discharge preparation method, or Aerosolpreparation method. In one embodiment, the method for making thesuper-aligned carbon nanotube array is chemical vapor deposition.

The solvent should have excellent wettability to the carbon nanotubes.The solvent can be ethanol, methanol, acetone, isopropanol,dichloroethane, chloroform, or the like. In one embodiment, the solventis ethanol, and the ethanol is placed in a wide mouth container, such asa beaker.

Titanium salt is used for hydrolytic reaction to generate the pluralityof titanium dioxide nanoparticles. The titanium salt can be butyltitanate, titanium tetrafluoride (TiF₄), titanium tetrachloride(TiFCl₄), titanium isopropoxide (Ti{OCH(CH3)2}4), or titanyl sulfate(TiOSO₄). In one embodiment, the titanium salt is tetrabutyl titanate(TBT).

In more detail, in step (S2), in one embodiment, the carbon nanotubearray is located on a substrate, and the carbon nanotube array can bescratched off from the substrate by a tool, such as a blade.

A time of ultrasonically dispersing the solvent having the plurality ofcarbon nanotubes can be selected according to actual needs, such as, asize of the carbon nanotube array and a power of the ultrasounddispersion. The three-dimensional network-like structure is a flocculentstructure. The carbon nanotubes in the flocculent structure areentangled with each other and uniformly distributed. The flocculentstructure is a porous fluffy structure, and a shape of the flocculentstructure like a batt in the traditional textile industry. During aprocess of ultrasonically dispersing the solvent having the plurality ofcarbon nanotubes, a power of ultrasonic waves can be in a range fromabout 400 W to about 1500 W. In some embodiments, the power ofultrasonic waves is in a range from about 800 W to about 1000 W. In oneembodiment, the power of ultrasonic waves is about 900 W, and the timeis about thirty minutes.

In one embodiment, in more detail, step (S3) further comprises adding analkaline solution, water or an acidic solution into the solvent with thethree-dimensional network-like structure. The alkaline solution can beammonia water. The acidic solution can be dilute sulfuric acid, dilutehydrochloric acid, acetic acid or the like.

Hydrolyzed titanium salt can be quantified, for example, by thefollowing two methods.

One method comprises adding the alkaline solution, water or acidicsolution into the solvent with the three-dimensional network-likestructure first, then adding the titanium salt into the solvent with thethree-dimensional network-like structure, wherein an amount of thetitanium salts exceeds an amount of the alkaline solutions, water oracidic solutions. With this method, the greater the amount of alkalinesolution, water or acidic solution, the greater the amount of titaniumdioxide nanoparticles are formed.

Another method comprises adding quantitative titanium salt into thesolvent with the three-dimensional network-like structure first, thenadding excess alkaline solution, water, or acidic solution into thesolvent with the three-dimensional network-like structure. “Excess”means that one of the reactants is in relative excess in a trimmedchemical reaction equation; there is still some residual reactant aftera complete reaction according to the proportional relationship in thetrimmed chemical reaction equation. An amount of alkaline solution,water, or acidic solution exceeds an amount of the quantitative titaniumsalt.

A size of each of the titanium dioxide nanoparticles can be controlledby the amount of the titanium salt, the alkaline solution, the water orthe acid solution, and a reaction time. The size of each of theplurality of titanium dioxide nanoparticles is less than or equal tothirty nanometers. In one embodiment, the size of each of the pluralityof titanium dioxide nanoparticles is larger than or equal to threenanometers and less than or equal to twenty nanometers. In anotherembodiment, the hydrolyzed titanium salt is quantified by the firstmethod, wherein a amount of the tetrabutyl titanate is 2.0 ml, a amountof the ammonia ranges from about 0.6 mL to about 1.4 mL, and thereaction time ranges from about one hour to about eight hours; and thesize of each of the plurality of titanium dioxide nanoparticles islarger than or equal to three nanometers and less than or equal to tennanometers.

In more detail, in step (S4), after adding the titanium salt into thesolvent with the three-dimensional network-like structure, the titaniumsalt hydrolyzes to form a plurality of titanium dioxide particles, andthe plurality of titanium dioxide particles are adsorbed on surfaces ofthe plurality of carbon nanotubes in the three-dimensional network-likestructure to form a mixture. The mixture consists of thethree-dimensional network-like structure and the plurality of titaniumdioxide particles. The solvent with the mixture is kept still orcentrifuged for some time, such as 1˜20 minutes; the mixture willdeposit to a bottom of the solvent, the solvent can be absorbed out froma container by a pipe, thereby separating the mixture from the solvent.The mixture can also be separated from the solvent by filtration, forexample, vacuum filtration or atmospheric filtration.

After the nanotube three-dimensional network structure adsorbed with theplurality of titanium dioxide nanoparticles are separated from thesolvent, the nanotube three-dimensional network structure adsorbed withthe plurality of titanium dioxide nanoparticles can be dried at a roomtemperature or at a temperature from about 30 centigrade to about 80centigrade. After the nanotube three-dimensional network structureadsorbed with the plurality of titanium dioxide nanoparticles is dried,the titanium dioxide-carbon nanotube composite film, the titaniumdioxide-carbon nanotube composite film can be cut directly to form thelithium-ion battery anode. In other embodiments, the titaniumdioxide-carbon nanotube composite film can be pressed and then cut toform the lithium-ion battery anode.

Referring to FIG. 10, One embodiment is described in relation to asecond method for making the lithium-ion battery anode 100. The secondmethod comprises at least the following general steps:

step (S′1), providing a carbon nanotube array on a substrate, a solventand a titanium salt;

step (S′2), scratching off the carbon nanotube array from the substrateto obtain a plurality of carbon nanotubes, adding the plurality ofcarbon nanotubes into the solvent, and ultrasonically dispersing thesolvent having the plurality of carbon nanotubes to make the pluralityof carbon nanotubes form a three-dimensional network-like structure;

step (S′3), adding the titanium salt into the solvent with thethree-dimensional network-like structure, and the titanium salt areadsorbed on surfaces of the plurality of carbon nanotubes in thethree-dimensional network-like structure; and

step (S′4), separating the nanotube three-dimensional network structureadsorbed with the titanium salt from the solvent, the titanium salthydrolyzes to form a plurality of titanium dioxide particles, and theplurality of titanium dioxide particles are adsorbed on surfaces of theplurality of carbon nanotubes in the three-dimensional network-likestructure to form a precursor, and drying the precursor to form atitanium dioxide-carbon nanotube composite film.

The second method is similar to the first method, except that thetitanium salt is not hydrolyzed in step (S′3); the titanium salthydrolyzes during or after a process of separating the nanotubethree-dimensional network structure adsorbed with the titanium salt fromthe solvent in step (S′4). The titanium salt can be hydrolyzed directlyin the air. The titanium salt can also be hydrolyzed by adding analkaline solution, water or an acidic solution.

The first method and the second method for making the lithium-ionbattery anode 100 can have many advantages. First, the lithium-ionbattery anode is prepared using a facile in-situ sol-gel method, suchmethod can be carried out at room temperature, and thus, the method issimple and easy. Second, the surface of the carbon nanotubes is notmodified by strong acids or surfactants during an implementation of themethod, thus avoiding an introduction of impurities and damage to thecarbon nanotubes. Third, the method does not need to add a binder, aconductive agent, and a current collector, thus, under the same specificcapacity and total capacity, the lithium-ion battery using thelithium-ion battery anode obtained by the method has a smaller mass;there are not insulation materials between the titanium dioxidenanoparticles, thus, the conductivity of the lithium-ion battery anodeis increased; and the conductivity of the lithium-ion battery anode ismore environmentally friendly. Finally, the lithium-ion battery anodeobtained by the method has good electrochemical performance without thebinder, the conductive agent, and the current collector.

Referring to FIG. 11, one embodiment of a flexible lithium-ion battery200 using the lithium-ion battery anode 100. The flexible lithium-ionbattery 200 comprises an external encapsulating shell 30, a lithium-ionbattery anode 100, a lithium-ion battery cathode 40, an electrolytesolution (not shown), and a separator 50. The lithium-ion batterycathode 40, the lithium-ion battery anode 100, and the separator 50 areencapsulated in the encapsulating shell 30. The electrolyte solution isfilled in the encapsulating shell 30. The separator 50 is locatedbetween the lithium-ion battery cathode 40 and the lithium-ion batteryanode 100. A cathode terminal 402 is electrically connected with thelithium-ion battery cathode 40. An anode terminal 102 is electricallyconnected with the lithium-ion battery anode 100. The flexiblelithium-ion battery 200 is a flexible structure. The flexiblelithium-ion battery 200 can be bent repeatedly without affecting aperformance of the flexible lithium-ion battery 200.

The lithium-ion battery cathode 40 can be a composite film comprising aplurality of cathode active material particles and a plurality of carbonnanotubes. The composite film is flexible and free-standing. Theplurality of carbon nanotubes are entangled with each other to form aporous film structure. The plurality of cathode active materialparticles are wrapped by the plurality of carbon nanotubes or attachedon the surface of the plurality of carbon nanotubes. Therefore, theplurality of carbon nanotubes can act as a conductive agent and play arole in fixing the plurality of cathode active material particles. Inone embodiment, the composite film consists of the plurality of cathodeactive material particles and the plurality of carbon nanotubes.

A material of the plurality of cathode active material particles can belithium iron phosphate (e.g., LiFePO₄), lithium nickel cobalt oxide(e.g., LiNi_(0.8)Co_(0.2)O₂), lithium nickel cobalt manganese oxide(e.g., LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂), lithium cobalt oxide (e.g.,LiCoO₂), lithium manganese oxide (e.g., LiMn₂O₄) or any combinationthereof. A shape of each of the plurality of cathode active materialparticles is not limited, and can be irregular or regular. A particlesize of each of the plurality of cathode active material particles canbe in a range from about 5 micrometers to about 20 micrometers. In oneembodiment, the material of the plurality of cathode active materialparticles is LiMn₂O₄, and the particle size of each of the plurality ofcathode active material particles is about 10 micrometers.

The plurality of carbon nanotubes can be single-walled carbon nanotubes,double-walled carbon nanotubes, or multi-walled carbon nanotubes. Aparticle size of each of the plurality of carbon nanotube can be in arange from about 1 nanometers to about 200 nanometers. The plurality ofcarbon nanotubes can be pure, meaning there are few or no impuritiesadhered on surface of the plurality of carbon nanotubes. In oneembodiment, a length of each of the plurality of carbon nanotube islonger than 300 micrometers, and the length of the plurality of carbonnanotube is equal.

A mass percentage of the plurality of cathode active material particlesin the composite film range from about 90 wt % to about 99.9 wt %. Insome embodiments, the mass percentage of the plurality of cathode activematerial particles in the composite film range from about 95 wt % toabout 99.9 wt %.

A method for making the composite film comprising the plurality ofcathode active material particles and the plurality of carbon nanotubescomprises at least the following steps:

step (M1): providing a carbon nanotube array, the plurality of cathodeactive material particles, and a solvent, wherein the carbon nanotubearray is located on a substrate;

step (M2): scratching off the carbon nanotube array from the substrateinto the solvent, dispersing the plurality of cathode active materialparticles into the solvent, and ultrasonically dispersing the solvent tomake the plurality of cathode active material particles and theplurality of carbon nanotube form a mixture; and

step (M3): separating the mixture from the solvent to form a precursor,and drying the precursor to form the composite film comprising theplurality of cathode active material particles and the plurality ofcarbon nanotubes.

The solvent is a non-aqueous solvent that can disperse the plurality ofcarbon nanotubes. The solvent can be ethanol, methanol, acetone,ethylene glycol, propanol, isopropanol, or the like. In one embodiment,the solvent is ethanol, and the ethanol is placed in a wide mouthcontainer, such as a beaker.

In more detail, in step (M3), the solvent with the mixture is keptstatic or centrifuged for some time, such as 1˜20 minutes; the mixturewill deposit to a bottom of the solvent, the solvent can be absorbed outfrom a container by a pipe, thereby separating the mixture from thesolvent. The mixture can also be separated from the solvent by keptstill or centrifuged for some time, the mixture will deposit to a bottomof the solvent, the solvent can be absorbed out from a container by apipe, thereby separating the mixture from the solvent. The mixture canalso be separated from the solvent by filtration, for example, vacuumfiltration or atmospheric filtration.

The composite film comprising the plurality of cathode active materialparticles and the plurality of carbon nanotubes can be cut directly toform the lithium-ion battery cathode. In other embodiments, the titaniumdioxide-carbon nanotube composite film can be pressed and then cut toform the lithium-ion battery anode.

Example 3

In the flexible lithium-ion battery of this example, the separator is aCelgard 2400 microporous polypropylene film. The electrolytic solutionis that 1 M LiPF6 is dissolved in a mixture of ethylene carbonate (EC),diethyl carbonate (DEC) and dimethyl carbonate (DMC). The plurality ofcathode active material particles are lithium manganate particles, thesize of each of the plurality of cathode active material particles isabout 10 micrometers. A mass percentage of the plurality of cathodeactive material particles in the lithium-ion battery cathode is about 96wt %. The particle size of each of the plurality of titanium dioxidenanoparticles is in a range from about three nanometers to about tennanometers. The mass percentage of the titanium dioxide nanoparticles inthe lithium-ion battery anode is about 28.7%. The mass density of thetitanium dioxide nanoparticles is about 1.3 mg/cm².

The flexible lithium-ion battery in Example 3 is tested for batteryperformance at room temperature with a cut-off voltage in a range from1.25 V to 3.0 V.

Referring to FIG. 12, when a cut-off voltage is in a range from 1.25 Vto 3.0 V, the flexible lithium-ion battery in Example 3 is discharged atcurrent densities of 5 C, 10 C, 20 C, 30 C and 60 C, respectively. Itcan be seen that the flexible lithium-ion battery in Example 3demonstrate discharge capacities of 150 mA h g⁻¹, 120 mA h g⁻¹, 95 mA hg⁻¹, 80 mA h g⁻¹, 50 mA h g⁻¹ at 5 C, 10 C, 20 C, 30 C and 60 C,respectively. Therefore, the flexible lithium-ion battery in Example 3has larger discharge capacity and smaller battery capacity decay ratio.

Referring to FIG. 13, the flexible lithium-ion battery in Example 3 canbe fully charged within 50 s, and an operating voltage of the flexiblelithium-ion battery is in a range from about 1.25 V to about 3.0 V.

Referring to FIG. 14, a capacity decay of the flexible lithium-ionbattery in Example 3 is almost negligible over 500 cycles of bendingwith a fast charging rate within 50 s, the capacity decay can reachabove 80%. It can be seen that bending does not reduce the capacity ofthe flexible lithium-ion battery.

The flexible lithium-ion battery 200 can have many advantages. First,the flexible lithium-ion battery 200 has flexibility and free-standingproperty, and can be bent freely; and the particle size of each of theplurality of titanium dioxide nanoparticles 20 is small, which let theflexible lithium-ion battery 200 has high capacities even at very highcharge/discharge rates. Second, the flexible lithium-ion battery 200without any binders, conductive additives or current collectors, thus,under the same specific capacity and total capacity, the flexiblelithium-ion battery 200 has a smaller mass; and the conductivity of theflexible lithium-ion battery 200 is higher. Third, the carbon nanotubefilm 10 comprises a plurality of micro pores, the plurality of micropores improves a penetration of the electrolyte and an ability of thelithium-ion battery anode 100 to adsorb and remove lithium-ions, and theplurality of micro pores can be used to hold lithium-ions, therefore, avolume of the flexible lithium-ion battery 200 will not be significantlyincreased in the process of using the flexible lithium-ion battery 200.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the present disclosure. Variations maybe made to the embodiments without departing from the spirit of thepresent disclosure as claimed. Elements associated with any of the aboveembodiments are envisioned to be associated with any other embodiments.The above-described embodiments illustrate the scope of the presentdisclosure but do not restrict the scope of the present disclosure.

Depending on the embodiment, certain of the steps of a method describedmay be removed, others may be added, and the sequence of steps may bealtered. The description and the claims drawn to a method may includesome indication in reference to certain steps. However, the indicationused is only to be viewed for identification purposes and not as asuggestion as to an order for the steps.

What is claimed is:
 1. A method for making a lithium-ion battery anodecomprising: step (S1), providing a carbon nanotube array on a substrate,step (S2), scratching off the carbon nanotube array from the substrateto obtain a plurality of carbon nanotubes, adding the plurality ofcarbon nanotubes into a solvent, and ultrasonically dispersing thesolvent having the plurality of carbon nanotubes to make the pluralityof carbon nanotubes form a three-dimensional network-like structure;step (S3), adding a titanium salt into the solvent with thethree-dimensional network-like structure, wherein the titanium salthydrolyzes in the solvent with the three-dimensional network-likestructure, to form a plurality of titanium dioxide nanoparticlesadsorbed on surfaces of the plurality of carbon nanotubes in thethree-dimensional network-like structure; and step (S4), separating thethree-dimensional network structure with the plurality of titaniumdioxide nanoparticles from the solvent to form a precursor, and dryingthe precursor to form the lithium-ion battery anode.
 2. The method ofclaim 1, wherein the titanium salt is butyl titanate, titaniumtetrafluoride, titanium tetrachloride, titanium isopropoxide, or titanylsulfate.
 3. The method of claim 1, wherein step (S3) further comprisesadding an alkaline solution, an aqueous solution, or an acidic solutioninto the solvent with the three-dimensional network-like structurebefore adding the titanium salt into the solvent with thethree-dimensional network-like structure.
 4. The method of claim 3,wherein the alkaline solution, the aqueous solution, or the acidicsolution is added into the solvent with the three-dimensionalnetwork-like structure before adding the titanium salt into the solvent;and then after the alkaline solution, the aqueous solution, or theacidic solution is added to the solvent, the titanium salt is added intothe solvent with the three-dimensional network-like structure, whereinan amount of the titanium salts exceeds an amount of the alkalinesolution, the aqueous solution or the acidic solution.
 5. The method ofclaim 4, wherein: the alkaline solution is about 0.6 mL to about 1.4 mLof ammonia; the titanium salt is about 2.0 ml of tetrabutyl titanate;and a reaction time for the titanium salt hydrolyzing in the solventwith the three-dimensional network-like structure is in a range fromabout 1 hour to about 8 hours.
 6. The method of claim 3, wherein anamount of the titanium salt is added into the solvent with thethree-dimensional network-like structure before the titanium salthydrolyzes; and then adding the alkaline solution, the aqueous solution,or the acidic solution into the solvent with the three-dimensionalnetwork-like structure and the titanium salt, wherein an amount of addedalkaline solution, the aqueous solution, or the acidic solution exceedsthe amount of titanium salt.
 7. The method of claim 1, wherein step (S3)further comprises adding ammonia to the solvent with thethree-dimensional network-like structure and the titanium salt beforethe titanium salt hydrolyzes.
 8. The method of claim 1, wherein step(S3) further comprises adding an acidic solution to the solvent with thethree-dimensional network-like structure and the titanium salt beforethe titanium salt hydrolyzes, and the acidic solution is dilute sulfuricacid, dilute hydrochloric acid, or acetic acid.
 9. The method of claim1, wherein in step (S3), a size of each of the plurality of titaniumdioxide nanoparticles is 3 nanometers-10 nanometers.
 10. The method ofclaim 1, wherein during a process of ultrasonically dispersing thesolvent having the plurality of carbon nanotubes, a power of ultrasonicwaves is in a range from about 400 W to about 1500 W.
 11. The method ofclaim 1, wherein the drying the precursor is carried out at a roomtemperature or between about 30° C. to about 80° C.
 12. A method formaking a lithium-ion battery anode comprising: step (S′1), providing acarbon nanotube array on a substrate; step (S′2), scratching off thecarbon nanotube array from the substrate to obtain a plurality of carbonnanotubes, adding the plurality of carbon nanotubes into a solvent, andultrasonically dispersing the solvent having the plurality of carbonnanotubes to make the plurality of carbon nanotubes form athree-dimensional network-like structure; step (S′3), adding a titaniumsalt into the solvent with the three-dimensional network-like structure,the titanium salt adsorbing on surfaces of the plurality of carbonnanotubes in the three-dimensional network-like structure; step (S′4),separating the nanotube three-dimensional network structure adsorbedwith the titanium salt from the solvent, the titanium salt hydrolyzes toform a plurality of titanium dioxide particles adsorbed on surfaces ofthe plurality of carbon nanotubes in the three-dimensional network-likestructure to form a precursor, and drying the precursor to form thelithium-ion battery anode.
 13. The method of claim 12, wherein thetitanium salt is butyl titanate, titanium tetrafluoride, titaniumtetrachloride, titanium isopropoxide, or titanyl sulfate.
 14. The methodof claim 12, wherein the titanium salt hydrolyzes during or after aprocess of separating the nanotube three-dimensional network structureadsorbed with the titanium salt from the solvent in step (S′4).
 15. Themethod of claim 12, wherein the titanium salt hydrolyzes directly inair.
 16. The method of claim 12, wherein the titanium salt hydrolyzes byadding an alkaline solution, an aqueous solution or an acidic solution.17. The method of claim 16, wherein the alkaline solution is ammonia.18. The method of claim 12, wherein in step (S′4), a size of each of theplurality of titanium dioxide nanoparticles is larger than or equal to 3nanometers and less than or equal to 10 nanometers.